1. Field of the Invention
The present invention relates to integrated circuit packages employing voltage reference planes and, more specifically, to integrated circuit packages disposing a ground plane or other voltage reference plane in close, substantially coextensive relationship with leads of a lead frame connected to a semiconductor die bearing an integrated circuit.
2. State of the Art
There is a continued trend in the computer industry toward ever-higher speed integrated circuit (IC) assemblies based upon semiconductor die technology. Such high signal speeds, however, lack utility unless accompanied by suppression of system noise to an acceptable level. The trend toward lower operational signal voltages in combination with such high speeds exacerbates noise problems.
At state-of-the art operational speeds, signal propagation delays, switching noise, and crosstalk between signal conductors resulting from mutual inductance and self inductance phenomena of the conductive paths all become significant to signal degradation. Mutual inductance results from an interaction between magnetic fields created by signal currents flowing to and from a lead frame-mounted, packaged IC die through the leads or "lead fingers," while self inductance results from the interaction of the foregoing fields with magnetic fields created by oppositely-directed currents flowing to and from ground.
Therefore, the integrated circuits carried on a semiconductor die would ideally be electrically connected to conductive traces on carrier substrates such as printed circuit boards and thus to other integrated circuits carried on the same or other such substrates by infinitesimally short conductors, eliminating impedance problems such as undesirable inductance and other conductor-induced system noise.
As a practical matter, however, as the capacity and speed of many integrated circuit devices such as dynamic random access memories (DRAMs) have increased, the number of inputs and outputs (I/Os) to each die has increased, requiring more numerous and complex external connections thereto, and, in some instances, requiring undesirably long lead frame lead fingers to place the inner lead ends in contact with, or in close proximity to, the bond pads serving as I/Os for the typical die.
While lead inductance in IC packages has not traditionally been troublesome because slow signal frequencies of past devices render such inductance relatively insignificant, faster and ever-increasing signal frequencies of state-of-the-art electronic systems have substantially increased the practical significance of lead inductance. For example, at such faster signal frequencies, performance of IC dice using lead frames for external electrical connection is slower than desirable because the inductance associated with the lead fingers slows changes in signal currents through the leads, prolonging signal propagation through the leads. Further, digital signals propagating along the lead fingers are dispersing or "spreading out" because the so-called "Fourier" components of various frequencies making up the digital signals propagate through the inductance associated with the lead fingers at different speeds, causing the signal components, and thus the signals themselves, to disperse along the lead fingers. While mild dispersion merely widens the digital signals without detrimental effect, severe dispersion can make the digital signals unrecognizable upon receipt. In addition, so-called "reflection" signals propagating along the lead fingers as a result of impedance mismatches between the lead fingers and associated IC die or between the lead fingers and external circuitry, caused in part by lead-associated inductance, can distort normal signals propagating along the lead fingers concurrently with the reflection signals. Further, magnetic fields created by signal currents propagating through the lead-associated inductance can induce currents in adjacent lead fingers, causing so-called "crosstalk" noise on the latter. While these various effects might be troublesome in any electronic system, the aforementioned trend toward lower voltage systems (currently 3.3 volts) and away from the traditional 5.0 volt systems increases their visibility and significance.
Certain currently-popular die and package configurations serve to exacerbate the noise problems by favoring a large plurality of laterally adjacent lead fingers of substantial length. For example, so-called lead-over-chip (LOC) configurations typically place the bond pads of a die in one or two rows extending along the longitudinal axis of the die. To accommodate the centralized bond pad location for wire-bonding and at the same time eliminate the need for a conventional die-attach paddle as a physical die support, LOC lead frames have been developed which employ lead fingers extending from the sides of the die and over the active surface into close proximity with the bond pad row or rows. The die is then supported from the undersides of the extending lead fingers, typically through an intervening polyimide film such as a Kapton.TM. tape having an adhesive coating on its upper and lower surfaces, the film serving as a dielectric, an alpha barrier and a protective coating for the active surface.
While a mechanically desirable packaging concept, the LOC-type long, mutually parallel lead finger runs over the active surface become abusive in terms of unacceptably increasing real impedance as well as lead inductance (both self and mutual) in the circuit. These lead finger runs also increase signal reflection in the circuit due to transmission line effects and degrade signal integrity due to the aforementioned propagation delays, switching noise, and crosstalk. Further, elimination of the die-attach paddle also eliminates the potential for employing a ground plane under the die without additional processing steps, and such a ground plane in any case would not alleviate the problems attendant to use of the long lead fingers extending over the die's active surface.
LOC configurations are merely one example of the type of packaging promoting the above-referenced undesirable noise phenomena. However, the same undesirable characteristics may be experienced with other lead frame configurations employing extended lead fingers, particularly large groups of such lead fingers in close mutual proximity. Such configurations include lead-under-chip (LUC) configurations, and configurations wherein a large number of leads extend from several sides of a semiconductor die to a single side or edge of a package, such as in a vertical surface mount package, or VSMP.
Packages have previously been configured in an attempt to reduce package noise of the type described above. For example, U.S. Pat. No. 5,214,845, assigned to the assignee of the present invention, employs a flexible, laminated sandwich assembly of an outer ground plane and an outer power plane dielectrically isolated from a series of conductive traces running therebetween. The traces and planes are connected to corresponding bond pads on the die at one end, and to lead fingers on the other, as by thermocompression bonding (in the case of a TAB embodiment) or by wire bonds. Such an arrangement obviously doubles the number of required I/O connections, necessitating additional fabrication time and increasing the possibility of a faulty connection. Further, the flexible sandwich assembly constitutes an additional element of the package, increasing material cost.
Another approach to the problem is disclosed in U.S. Pat. No. 5,559,306, wherein metal plates are employed above and below leads extending to the exterior of plastic and ceramic packages to effect reduction of self and mutual inductance. However, such configurations as disclosed appear to require relatively complex fabrication techniques to locate and fix the plates relative to the die and lead fingers or other conductors for subsequent transfer molding of a filled-polymer package thereabout, while the ceramic package embodiment is not cost-effective for high-volume, commercial packaging.
Accordingly, the inventors have recognized the need for a low-cost, reduced-inductance circuit configuration adaptable to current packaging designs and employing conventional and readily-available materials, equipment and fabrication techniques.